Simplifying the expression:
\[
(6 + 5\sqrt{3}) - (4 - 3\sqrt{3}) = (6 - 4) + (5\sqrt{3} + 3\sqrt{3}) = 2 + 8\sqrt{3}
\]
Since \(\sqrt{3}\) is an irrational number, multiplying it by 8 still results in an irrational number. Hence, the result is an irrational number.